Method for regulating a process for manufacturing paper pulp by measuring the amount of hexenuronic acid optically

ABSTRACT

Method for enabling the paper pulp production process to be controlled in such a way that the hexenuronic acid content in the pulp can be minimized and the colour reversion of the pulp and/or the pulp products thereby minimized. The method is based on measurements of the optical properties of a fibre-containing substance or process stream in the wave length region 232±5 nm and another wavelength interval. A significant value for the current hexenuronic acid content is formed by correcting the intensity in the spectrum around 232 by the corresponding optical quantity at the other wavelength interval. The content of the hexenuronic acid content which is determined by the analysis is then used for controlling conditions in the process stage.

TECHNICAL FIELD

[0001] The invention relates to a method for regulating a process formanufacturing paper pulp in which the content of hexenuronic acid in thepaper pulp is determined during the manufacture of the paper pulp, inaccordance with the preamble to Patent claim 1.

STATE OF THE ART

[0002] Paper is a material which has many areas of application. Thedemand for different papers, and consequently for their principal rawmaterial paper pulp, are therefore very variable. The optical propertiesof the paper, such as its colour, are very important in many applicationareas. It can be mentioned that the paper pulp is sometimes stained bluein order to deceive the eye into believing that the paper is whiter,i.e. less yellow, than it in fact is.

[0003] The optical properties of a paper pulp are altered during thecourse of the production process when the wood is cooked in order toexpose the fibres and during bleaching. To a large extent, this is dueto the breakdown and removal of colouring and light-absorbing structures(chromophores) in the wood, such as lignin and certain carbohydrates andthe breakdown and hydrolysis products of these substances.

[0004] Some of these unwanted structures are always left in the paperpulp after cooking and have to be attacked in the subsequent bleaching.Apart from lignin, another important structure, namely the so-calledhexenuronic acid (also termed HexA below) has attracted ever-increasinginterest in recent years since it has been shown that when this acid isleft in the paper pulp, and hence the paper, it can inter alia causecolour reversion, i.e. meaning that the paper becomes yellower as itages.

[0005] Methods for reducing colour reversion caused by breakdownproducts which have not been washed out were presented at an early stagein SE225253, which presented acidic and basic treatments. Treatment inan acidic medium (pH of about 3) and for a relatively long period (2hours), and at a high temperature (80-120 degrees), has been found toresult in substantial reductions in the colour reversion of the pulp.

[0006] HexA has its origin in the xylan of the native wood, with xylanbeing a polymer which occurs with a particularly high degree offrequency in hard wood. HexA is a carbohydrate structure which, usingmore rational nomenclature, can be designated4-deoxy-β-L-threo-4-enopyranosyluronic acid. It is not present as anative component in the wood but is instead formed during alkaline paperpulp production from the 4-O-methylglucuronic acid structure which isbonded to the xylan.

[0007] Methods for decreasing the content of HexA by treating paper pulpare known, see, for example, T. Vuorinen et al. Selective Hydrolysis ofHexenuronic Acid Groups and its Application in ECF and TCF Bleaching ofKraft Pulps; J. Pulp Paper Science 25(5): 155-162 (1999). It is alsowell known that chlorine dioxide and ozone, inter alia, react with HexAduring bleaching. However, it is also known that the frequently employedbleaching chemicals oxygen and hydrogen peroxide, for example, do notdecompose HexA. It has not hitherto been possible to measure, in anysimple and automated manner, to what degree precisely HexA is attackedby different chemicals in connection with industrial pulp production,and it has not therefore been possible to follow the reduction of HexAin any controlled manner, to the detriment of optimizing processconditions which are selected in the bleaching department. Instead, ithas been necessary to rely on the very vague concept of kappa number,which does not differentiate between lignin and HexA.

[0008] Since there are no simple and rapid methods for analysing HexA,it is difficult, during industrial operation, to optimize and controlthe process with a view to decreasing the quantity of HexA over one ormore process stages, for example by varying the pH, the temperature andthe quantities of chemicals added. Without any doubt, this makes it moredifficult to manufacture a product which is both bright andbrightness-stable.

[0009] As a rule, methods for analysing HexA which have been describedto date require some form of preliminary hydrolysis, which can comprisetreating with acid in a pressure vessel at high temperatures (T.Vuorinen et al.; Selective Hydrolysis of Hexenuronic Acid Groups in ECFand TCF Bleaching of Kraft Pulps, International Pulp BleachingConference, Washington D.C., USA 1996, Vol. I 42-51), swelling the pulpfor several hours, followed by treating with mercuric acetate solutions(G Gellerstedt; Li, J; An HPLC Method for the Quantitative Determinationof Hexenuronic Acid Groups in Chemical Pulps, Carbohydr. Res. 1996, 29441-51), or various time-consuming enzymic treatments (M Tenkanen et al.;Use of Enzymes in Combination with Anion Exchange Chromatography in theAnalysis of Carbohydrate Composition of Kraft Pulps, 8th InternationalSymposium on Wood and Pulping Chemistry, Helsinki, Finland 1995, Vol.III 189-194); (A Rydlund; Dahlman, O; Rapid Analysis of UnsaturatedAcidic Xylooligosaccharides from Kraft Pulps using Capillary ZoneElectrophoresis (CZE), J. High. Resolut. Chromatogr. 1997, 20 (2),72-76). The chemical methods give relatively rapid hydrolysis and in themain use UV spectrophotometry (T Vuorinen et al., 1996) or HPLC (GGellerstedt and Li, J.; 1996) for the subsequent analysis. Thestructures which are analysed are in actual fact various breakdownproducts which are derived from the HexA structure but which can varydepending on the method by which the HexA has been broken down orhydrolysed. Thus, it is not possible, for example, to study the contentof a particular given breakdown product derived from HexA in the liquidphase from different bleaching stages and correlate this directlyagainst the quantity of HexA which has been broken down. The reactionproducts differ greatly from each other depending on whether thebreakdown has taken place by means of acidic hydrolysis, ozonolysis orreaction with chlorine dioxide, for example. Because of eithernonspecific breakdown products, complicated analytical methods, theamount of time required or complicated equipment, none of theabovementioned analytical methods is suitable, or even possible, toapply for at-line studies of the removal of HexA in a bleachingdepartment.

[0010] From a purely theoretical point of view, a simple analyticalmethod ought to be that of directly determining the HexA in the pulpspectrophotometrically since HexA has a specific absorption peak at 232nm. It ought therefore to be possible to exploit this for quantitativelydetermining HexA. However, other structures in the pulp, of which theforemost is lignin, also absorb strongly at this wavelength. It is nottherefore possible to use spectrophotometry directly for determiningHexA at this particular wavelength.

DESCRIPTION OF THE INVENTION

[0011] The purpose of the invention is to offer a simplified method fordetermining the decrease (change) in the quantity of HexA in paper pulpor paper between different measurement sites or at different measurementtimes in order, thereby, to be able to carry out continuous monitoringof, and adjustments to, the manufacturing process during its course.This then makes it possible to control the process and to optimizedifferent process variables in order to eliminate, or substantiallydecrease, the quantity of HexA with the aim of improving pulp qualityand the cost-effectiveness of the operation. This aim is achieved bymeans of a method as described in the characterizing part of Patentclaim 1.

[0012] The method according to the invention is based on opticalmeasurements of the absorbance, reflectance, transmittance or similaroptical property of pulp suspensions at at least two differentwavelengths and in such a way that the measurement of the opticalproperty in the wavelength region around 232 nm, which is characteristicfor HexA, is decreased by the value of the optical property in awavelength region which corresponds to the absorbance of otherchromophores, such as lignin, which are present, for example in theregion around 280 or 205 nm (if the quantity is not additive per se, itis firstly converted into an additive quantity). Measurements can alsobe performed in the whole wavelength range for the purpose ofmultivariant assessment. In every case, use is made of the differencesbetween two different pulp suspensions which are respectively removedbefore and after the treatment (for example bleaching) whose effect onthe content of HexA it is wished to study. Thus, if it is decided to useonly two different wavelengths for the calculation, the quantity of HexAwhich is removed over a given bleaching stage can be determined inaccordance with the following formula:

ΔHexA=k[(A ₂₃₂ −A ₂₈₀)_(entering)−(A ₂₃₂ −A ₂₈₀)_(leaving)]

[0013] where

[0014] k is a proportionality constant and the A values are theabsorbance, or some other additive optical quantity, at the givenwavelength, measured either directly or calculated from some knownrelationship, such as the Kubelka-Munk relationship.

[0015] Additional features and aspects, and also advantages, of theinvention will be evident from the subsequent patent claims and thefollowing example.

EXAMPLE WHICH DESCRIBES A LABORATORY APPLICATION OF THE INVENTION

[0016] Pulps having different contents of HexA were produced by means ofthe acidic hydrolysis of an oxygen-delignified kraft hardwood pulp, withthe pulp being treated in stainless steel autoclaves for 60 minutes at85, 95, 100 and 110° C. The autoclaves were maintained at a pressure of8 bar using nitrogen. The pH was adjusted to 3.2 by adding sulphuricacid. This acid treatment of the pulp is selective with respect to HexAas far as structures which are of importance to the method areconcerned.

[0017] The pulps were disintegrated in accordance with the SCAN-testmethod SCAN C 18:65, 1964: Disintegration of Chemical Pulp for Testing.Laboratory sheets were manufactured in accordance with the SCAN-testmethod: SCAN CM 11:95, 1995: Preparation of Laboratory Sheets (OpticalProperties). When using this standard method, the pH of the pulpsuspension was adjusted to 5±0.3 in order to provide standardizedconditions. The laboratory sheets were made using a sheet moulder andnot using a funnel, which is an alternative according to the standardmethod. The grammage of the sheets was 60 g/m², which is not inagreement with the standard method but which was necessary in order tomeet the requirements for the Kubelka-Munk theory to be applicable.Three sheets were made in the case of each pulp and each sheet was cutinto several pieces which were placed one on top of the other in a pilesuch that it was possible to use the rough side for measurements indiffuse, reflected light. The measurements on these sheet piles wereperformed using a Varian Cary 100 spectrophotometer equipped with anattachment for measuring in diffuse, reflected light, and theΔ(K/S)_(corrected) was calculated using the formulae$\frac{K}{S} = \frac{\left( {1 - R_{\lbrack{illegible}\rbrack}} \right)^{2}}{2\quad R_{\lbrack{illegible}\rbrack}}$

[0018] and

Δ(K/S)_(corrected)=Δ(K/S)_(232 nm)−Δ(K/S)_(280 nm)

[0019] where K/S is assumed to be proportional to the chromophoreconcentration due to the fact that the change in the scatteringcoefficient S through the bleaching department can be considered to benegligible.

[0020] In order to evaluate the validity of the above relationship, theliquid from each autoclave was collected, together with the filtrate andthe washing liquid from the pulp, and the solution was diluted to 2.0litres in a measuring cylinder. Quantitative analyses of HexA in thisliquid were performed as described by Vuorinen at al. (T Vuorinen etal.; Selective Hydrolysis of Hexenuronic Acid Groups in ECF and TCFBleaching of Kraft Pulps, International Pulp Bleaching Conference,Washington D.C., U.S.A. 1996, Vol. I 42-51). Since the above acidtreatment of the pulp is selected with respect to HexA, there should bea good correlation between the change in the content of HexA as measuredby the method described by Vuorinen et al. and by means of theabovementioned measurements and calculations of Δ(K/S)_(corrected),which proves to be the case (FIG. 1). The above relationship istherefore valid.

[0021] The differences between the example and the form which it ishereby sought to patent lie in the physical performance of themeasurement, with the method which it is sought to patent being based onmeasurements using the type of optical instruments which have forseveral years been located in many mills for the purpose of determiningkappa number at-line or on-line.

[0022] The method according to the invention provides a procedure whereit is possible, in a feed-back manner, to optimize the process withregard to reducing hexenuronic acid. The process conditions such astemperature, acid composition, addition of chemicals, dwell time orpressure can then be adjusted such that the desired reduction isobtained. This approach can be applied irrespective of the type ofprocess stage, which can be an A stage (acid stage), D stage (chlorinedioxide stage), Z stage (ozone stage), C stage (chlorine stage), Paastage (peracetic acid stage), Ca stage (Caros acid stage) or anotherstage where the conditions lead to a reduction in hexenuronic acid.

1. Method for regulating a process for manufacturing paper pulp, wherethe content of hexenuronic acid in the paper pulp is determined duringmanufacture of the paper pulp, characterized in that the currentproportion of hexenuronic acid in the substance obtained from theprocess stage, which substance, for example a pulp suspension, containsat least a proportion of pulp fibres which have passed through theprocess, and where the current proportion of hexenuronic acid isdetermined by optical spectral analysis by way of measurements at atleast two different wavelengths, where the intensity in the spectrumaround 232 nm constitutes a significant value for the current content ofhexenuronic acid in the paper pulp, which significant value is correctedin dependence on an optical property in another wavelength interval,with the result, in the form of a corrected significant value from thecontent of hexenuronic acid, as determined by the optical analysis,being used to control the conditions in the process stage from which thepulp is obtained.
 2. Method according to claim 1, characterized in thatthe optical analysis is performed both before and after the processstage, between different measurement points, for example over one ormore bleaching stages, or at different measurement times, and in thatthe change in the intensity in the spectrum around 232 nm between thesemeasurement points or measurement times is used to control conditions inthe process stage from which the pulp is obtained.
 3. Method accordingto claim 1 or 2, characterized in that optical measurements areperformed at at least two different wavelengths and differences in someadditive optical quantity between before and after a given treatment arecalculated, and where the change in question is proportional to thequantity of HexA—in some cases directly, but especially only aftercorrecting for the change in the optical property in another wavelengthinterval.
 4. Method according to claim 3, characterized in that thecorrection according to claim 3 is determined by measuring the sameadditive optical property which was used in claims 1-3 for directlydetermining the hexenuronic acid content, where a correction term forthe correction is determined by optically measuring with dominancearound at least one of the wavelengths 205 nm or 280 nm.
 5. Methodaccording to claim 3 or 4, characterized in that the change in thehexenuronic acid content is determined in accordance with the formula:ΔHexA=k[ΔX _(232±5 nm) −ΔX _(205±5 nm or 280±5 nm)] where X denotes agiven additive optical quantity and k is a proportionality constant. 6.Method according to any one of Patent claims 1-5, characterized in thatthe said optical measurements are performed on a suspension of paperpulp.
 7. Method according to any one of Patent claims 1-5, characterizedin that the said optical measurements are performed on webs or sheets ofpaper pulp.
 8. Method according to any one of Patent claims 1-5,characterized in that the said optical measurements are performed onwebs or sheets of paper.
 9. Method according to any one of the precedingclaims, characterized in that the paper pulp is in the main producedfrom hard wood.
 10. Method according to any one of the preceding claims,characterized in that at least one of the process parameters dwell time,temperature, pressure, chemical addition or acid addition is adjusted independence on the result from the optical analysis.